One of the challenges in designing permanent magnet electrical machines, i.e. permanent magnet motors or generators, is related to fixing of permanent magnets to a rotor of a permanent magnet electrical machine. Usually the construction of the rotor is a compromise with respect to several factors such as, for example: cooling of the permanent magnets, mechanical strength especially against centrifugal forces, hermetic protection against moisture and other environmental factors, utilization of the material of the permanent magnets without e.g. excessive leakage magnetic fluxes via e.g. supporting structures, simplicity/complexity of the shape of the permanent magnets, fluctuations of produced torque, and factors related to installation and manufacture.
Publication EP1860755A2 discloses a permanent magnet rotor arrangement that includes a rotor having an outer rim. A circumferential array of magnet carriers is affixed to a surface of the outer rim. Each magnet carrier has a surface facing towards the radial direction. An inverted U-shaped retainer made of nonmagnetic material such as stainless steel or glass-reinforced epoxy resin is affixed to each magnet carrier and is arranged to form together with the magnet carrier an axially extending channel. At least one piece of permanent magnet material is located in the axially extending channel. The magnet carrier, the inverted U-shaped retainer, and the at least one piece of permanent magnet material constitute a permanent magnet module that extends over 360/N degrees in the tangential direction of the rotor, where N is the number of permanent magnet modules that are located successively in the tangential direction. Permanent magnet modules of the kind described above can be produced separately and used as building blocks of a permanent magnet rotor. Spatial distribution of magnetic flux density in an air-gap of a permanent magnet electrical machine has to be sufficiently close to a desired form in order to keep fluctuations of torque produced by the permanent magnet electrical machine at a sufficiently low level. In conjunction with permanent magnet modules of the kind described above, achieving a desired spatial distribution of the magnetic flux density requires shaping of the permanent magnets. For example, rectangular prism shaped permanent magnets do not necessarily provide a sufficiently good quality of operation. Therefore, there is a trade-off between the complexity of the shape of the permanent magnets and the quality of operation. The need for shaping the permanent magnets makes the permanent magnets more expensive and more difficult to manufacture.